1 Equipment Selection

1.1 Selection of Melting Equipment The selection of melting equipment should first meet the production requirements and follow the principle of low consumption. The advantage of an induction furnace is that it has a fast heating speed, high thermal efficiency, less oxidation loss, and less absorption of gases. Therefore, using a medium-frequency furnace for melting can avoid the problem of increased sulfur and phosphorus, and ensure that the P content in the iron water is no more than 0.07% and the S content is no more than 0.05%.

1.2 Determination of the Spheroidizing Ladle To improve the absorption rate of the spheroidizing agent and enhance the spheroidization effect, the spheroidizing treatment ladle should be deeper than the ordinary iron liquid ladle. The height-to-diameter ratio of the spheroidizing ladle is determined to be 2:1.

2 Raw Material Selection

2.1 Selection of Raw Materials The addition effect of the spheroidizing agent for ductile iron is conditional: high carbon, low silicon, and large malleability. To stabilize the chemical composition and effectively control the promoting elements of white cast iron and anti-spheroidization elements, ensuring the quality of the molten iron during melting, Tanger Steel Z14 pig iron is selected. Its chemical composition: C > 3.3%, Si 1.25% - 1.60%, P ≤ 0.06%, S ≤ 0.04%.

2.2 Selection of Spheroidizing Agent The selection of spheroidizing agent should be based on the different melting equipment, that is, the tapping temperature and the purity of the iron liquid (such as sulfur content, oxidation degree, etc.). The commonly used in China is rare earth magnesium silicon iron spheroidizing agent. When treating with this spheroidizing agent, due to the high silicon content in the alloy, the intensity of the reaction during magnesium treatment can be significantly reduced. At the same time, it also has some malleability enhancement effect due to the increase in silicon. In electric furnace production, because the temperature is relatively high, the spheroidizing agent used is FeSiMg8Re7.

3 Furnace Front Control

3.1 Chemical Composition Selection The original iron liquid for ductile iron should be high carbon, low silicon, low sulfur, and low phosphorus. Controlling the sulfur content well is an important condition for producing ductile iron.

3.2 Spheroidization and Malleation Treatment The amount of spheroidizing agent addition should be determined by analyzing and comparing factors such as the composition of the iron liquid, the wall thickness of the casting, the composition of the spheroidizing agent, and the absorption rate during the spheroidization treatment process. Generally, it is 1.6% - 2.0%. If the spheroidizing agent is placed for a long time, it should be added in an appropriate amount. The key to controlling the spheroidization reaction is the absorption rate of magnesium. High temperature, intense reaction, short time, more magnesium loss, poor spheroidization effect; low temperature, stable reaction, long time, high magnesium absorption rate, good spheroidization effect. Therefore, generally, under the premise of ensuring sufficient pouring temperature, it is advisable to reduce the spheroidization treatment temperature as much as possible, controlling it within 1420 - 1450℃. The spheroidizing agent should be crushed into small pieces, with a particle size generally of 5 - 25mm, added to the bottom of the ladle, and then on top, add silicon iron and iron chips.

The spheroidization treatment is an important link in the production of ductile cast iron. It not only promotes graphite formation, prevents the appearance of free carbides and white cast iron, but also helps spheroidization and makes the graphite finer, rounder, and more evenly distributed, thereby improving the mechanical properties of ductile cast iron. The spheroidization agent is generally FeSi75, and its addition amount is generally 0.8% - 1.0% according to the mechanical property requirements of the casting. The particle size of the spheroidization agent is generally crushed into 5 - 25mm pieces. The spheroidization agent should be kept clean and dry.

Spheroidizing agent and spheroidization agent should be added to the ladle before tapping. During continuous production, after the previous batch of iron is tapped, the ladle is very hot. Adding it too early will cause it to stick to the bottom of the ladle and weaken the spheroidization and spheroidization effect. To delay the spheroidization reaction time and enhance the spheroidization and spheroidization effect, a layer of iron chips should be covered on top of the spheroidizing agent and spheroidization agent. There are many methods for spheroidization treatment, and the most commonly used is the simple operation of the injection method for treating ductile iron.

3.3 Spheroidization Effect Furnace Front Inspection To check the good or bad spheroidization and spheroidization effect, a triangular sample is generally used. Pour the triangular sample, cool it to dark red, quench it with water, and break it to observe the fracture. The fracture is silver-white, with white cores at the tip, with porosity, concave contraction on both sides, and there is an electric stone smell when breaking, and the knocking sound is similar to steel. Then the spheroidization is good; otherwise, it is poor. 3.4 Pouring Since the liquid of ductile iron is prone to balling degradation, it is necessary to pour the iron liquid as soon as the balling treatment is completed. Generally, the pouring should be finished within 15 minutes after the treatment, and there will be no problem of balling degradation.